Cementation processes are known for recovering metal from aqueous solutions containing ions of the metal. In the cementation process, metal, referred to as the reducing metal, having a more negative standard state reduction potential than the metal to be recovered is contacted with the solution containing ions of the metal to be recovered. The recovered metal precipitates from solution onto the surface of the reducing metal, and the reducing metal enters into solution. Cementation, however, has some serious disadvantages for practical operation. Firstly, to obtain substantial removal of metal in a reasonably short time, the solution must be agitated to assure intimate exposure of the metal ions to the reducing metal surface. Since the reducing metal is generally present as pieces with relatively large dimensions, such as nuggets, shot, or pieces of scrap, it is difficult or impractical to suspend the reducing metal in the solution. Simply allowing the solution to flow over or up through the reducing metal pieces leads to channeling and short-circuiting, or requires an extremely large excess of reducing metal to obtain substantial removal of the metal in solution. Secondly, if only small beads or granules of the reducing metal are used, to allow easier suspension by agitation and to provide higher surface area for reduction, it becomes difficult to physically separate the precipitated metal from the diminishingly small particles of reducing metal.
Another known method for removing metal from solution is electrowinning, wherein a direct current is applied across the anodes and cathodes, precipitating the metal to be removed at the cathode. However, this method has the disadvantage that direct current must be provided by an expensive rectifier. Furthermore, the required oxidation reaction at the anode adds substantially to the power required to operate the electrowinning cell. This reaction is typically the formation of oxygen from water. But in a frequently occurring situation, where chloride ion is present, the undesireable side reaction of chloride oxidation can take place, leading to the formation of toxic chlorine gas.
It would be possible to afix pieces of reducing metal having high surface area, such as screens or thin sheets, in a vessel containing the solution of the metal to be recovered, and the metal would rapidly precipitate out as the solution is pumped through the vessel, providing high recovery rates of the metal. But the pieces of reducing metal would also dissolve, rapidly losing the very surface area that was the source of the high recovery rates. Since high surface area reducing metal is expensive, as would be the maintenance cost of frequently replacing it, this cementation system is not an attractive option. Alternatively, if larger pieces of reducing metal are used so that they last longer, they do not provide the high surface area needed for rapid removal rates.
It would be desireable to have a metal recovery apparatus and process which (1) does not require intensive agitation to give intimate contact of the solution with the reducing surface, but simply requires gentle pumping of the solution through the apparatus, (2) does not require rectified direct current and does not generate chlorine gas, (3) gives rapid removal of metal from solution, and (4) requires minimal maintenance and operator attention.
The present invention provides an apparatus and process in which the metal is recovered at a different portion of the apparatus than where the reducing metal is passing into solution. It provides a high surface area for efficient removal of metal without requiring high surface area reducing metal. The apparatus provides for rapid removal of the metal from the solution, without the need for frequent replacement of the reducing metal. The apparatus also provides means by which the recovered metal can be rapidly separated from the solution.